The Ultimate Guide to Sterilizing Mushroom Substrate for Beginners

The Ultimate Guide to Sterilizing Mushroom Substrate for Beginners

The cultivation of mushrooms at home is an enriching experience, offering fresh and delectable gourmet mushrooms that also offer numerous health benefits.


For novices, mushroom grow kits provide an exceptional starting point, and easy-to-cultivate varieties such as oyster mushrooms can be grown with minimal equipment. However, if you wish to cultivate more delicate or medicinal mushrooms that require nutrient-rich substrates, sterilizing your mushroom substrate is essential.


It is crucial to understand the difference between sterilization and pasteurization and how to sterilize your mushroom substrate using a pressure cooker. In the following sections, we will delve into these topics and provide step-by-step guidance on the sterilization process.


Unpacking Sterilization


To prepare mushroom substrates, sterilization is a method that utilizes steam, pressure, time, and temperature to eradicate living organisms and spores.


Achieving a substrate sterilization requires a temperature of at least 250°F (121°C) for a minimum of 2 hours, which cannot be accomplished by boiling water or steam alone.


Water boils at 212°F (100°C) at sea level, and increasing the heat does not raise the temperature any higher. Thus, pressure needs to be added to elevate the temperature to 250°F (121°C) or above to eliminate bacteria and fungal spores.


Typically, a pressure of 15 pounds per square inch (PSI) is required to reach this temperature. Mushroom cultivators typically use a pressure cooker for sterilization on a small scale and switch to an autoclave, retort, or pressure canner as their mushroom business expands.


Is Sterilizing Mushroom Substrate Necessary?


The requirement for substrate sterilization depends on the nutritional content and the type of mushrooms one aims to grow. Substrates with a high level of supplementation, such as manure, soy hulls, grains, and master's mix, as well as nutrient-rich substrates, are more prone to contamination.


These moist and nutrient-rich substrates attract not only mushroom mycelium but also other organisms like bacteria and mold, which can grow faster than the mushroom mycelium. Therefore, sterilizing these substrates becomes essential as it eradicates all living and dormant organisms and fungal spores, providing the best possible start for mushroom mycelium.


However, low-nutrient substrates like straw, sugarcane bagasse, coco coir, hardwood sawdust, and cardboard only require pasteurization. Our article on mushroom substrates offers additional information on different types of substrates and how to prepare them for cultivation.


Mushroom Substrate: Sterilization vs. Pasteurization


The primary contrast between sterilization and pasteurization lies in their respective goals. While sterilization aims to eliminate all potential contaminants, pasteurization only eliminates most of the competing organisms.


There are various pasteurization methods that effectively reduce the number of competing organisms in low-nutrient substrates like straw and sugarcane bagasse, which provides fast-growing mushroom species an advantage.


However, for slow-growing mushroom species that require more nutritious substrates or longer colonization times, the best approach is to use sterilized substrates with minimal competition.


Sterilizing Mushroom Substrate Using a Pressure Cooker


For those with some experience growing mushrooms who are interested in trying their hand at cultivating more difficult gourmet or medicinal species, sterilizing the substrate is a necessary step.


The most straightforward method for small-scale growers is using a pressure cooker that can maintain 15 PSI of pressure. To begin, gather the following items: a pressure cooker, jars or bags of prepared substrate, a wire rack or jar lids for the bottom of the pressure cooker, and water.


Mushroom grow bags are recommended since they are designed to withstand high sterilization temperatures and make growing mushrooms easier. However, jars may also be used for the substrate. Follow the steps below to safely sterilize the mushroom substrate:


Step 1: Ensure Your Pressure Cooker is Ready


Before using a pressure cooker for sterilizing mushroom substrate, it is essential to inspect it thoroughly to make sure all components are functioning properly. Look for any signs of damage such as dents, bulges, or cracks.


Some pressure cookers have rubber seals that may degrade over time, resulting in leaks. Check the seals carefully for any signs of damage.


Examine the cooker lid and ensure that nothing is blocking the vent pipe. Confirm that all screws are securely tightened and that the pressure gauge reads zero and is free of damage.


Step 2: Transfer the mushroom substrate into the pressure cooker


To prevent excess moisture from entering the substrate, you should fold mushroom grow bags to prevent steam from entering through the filter patch and cover any jar filters with foil.


Placing a metal rack or jar lids at the bottom of the pressure cooker and putting substrate jars or bags on top helps prevent melting or cracking from direct contact with the cooker's base.


Make sure to avoid having the bags or jars touch the sides of the pot, as it can cause damage to the containers. If necessary, line the lower sides of the pot with a cloth or kitchen towel.


Step 3: Make sure you add enough water


To ensure that there is enough water for the entire sterilization process, you will need to add the appropriate amount of water to the pressure cooker.


Typically, it is recommended to add approximately 3 quarts (2.8 liters) of water, or enough water to cover the substrate bags or jars up to 1 to 2 inches (2.5 to 5 cm) from the sides.


As you gain more experience, you will be able to determine the exact amount of water required, but in the beginning, it is important to monitor and take notes.


Before you close the lid of the pressure cooker, make sure to weigh down any grow bags with a heavy plate or similar object. This is necessary to avoid the bags from obstructing the pressure relief valve on the lid of the pressure cooker.


Step 4: Seal the lid and turn up the heat


To begin the sterilization process, put the lid on the pressure cooker and make sure it’s sealed tightly. Then, put the pressure cooker on the stove or gas cooker and turn the heat up to the maximum setting.


It may take some time for the water to boil and heat up the pressure cooker. When the pressure cooker gets hotter and the pressure builds up, steam will start to come out of the vent pipe.


Once the steam is coming out steadily and no longer spluttering, place a 15 PSI pressure regulator weight on the vent to regulate the pressure.


Step 5: Monitor Pressure and Sterilize the Substrate


As heat continues to be applied, pressure within the pressure cooker will gradually increase until it reaches 15 PSI, causing the pressure regulator weight to move and let out steam.


Once this occurs, the heat should be adjusted until the steam has just enough pressure to cause the weight to move gently up and down.


Once you have achieved the right pressure, set a timer or alarm to keep track of your sterilization time.


Step 6: Remove from heat and cool


After the sterilization process is complete, turn off the heat and leave the substrate to cool inside the pressure cooker for at least 8 hours.


Once the substrate is completely cool, it is ready to be inoculated with mushroom spawn. It's recommended to do this step in front of a laminar flow hood to minimize contamination.


Can substrate be sterilized with a pressure cooker?


Mushroom growers commonly use pressure cookers or autoclaves to sterilize their substrates since high temperatures above 250°F (121°C) cannot be reached without pressure.


However, it is possible to achieve similar results without a pressure cooker. To do this, growers must maintain high temperatures for longer periods, typically accomplished using a barrel steam sterilizer. These are ideal for small mushroom farmers who may not have the funds for an autoclave.


Since they operate without pressure, barrel steam sterilizers are less expensive, and it's possible to build one yourself. If you prefer to buy one, you can find ready-to-use barrel steam sterilizers online.


When using a steam sterilizer, the substrate is super pasteurized, which involves keeping it as close to 212°F (100°C) as possible for long periods. With a barrel steam sterilizer, the substrate must be kept between 194 and 212°F (90 and 100°C) for up to 18 hours, taking much longer than classic sterilization but producing nearly identical results and suitable for most mushroom species.


Can You Sterilize Mushroom Substrate in an Oven?


Sterilizing mushroom substrate in the oven is not possible, but it is possible to pasteurize it. This method is sometimes used for small amounts of substrate or casing material.


However, there are several challenges to consider when pasteurizing mushroom substrate in the oven. For example, you can’t use bags, so the substrate is often exposed to airborne contaminants when it cools down.


In addition, the oven’s dry heat may dry out and burn the substrate, reducing the nutrients available for the mycelium. If the substrate dries out too much, you’ll need to rehydrate it with distilled water to help prevent contamination.


Does sterilizing in the microwave work?


Microwaving can be used to pasteurize small amounts of mushroom substrate, but it is not an effective method for sterilization since it cannot eliminate all fungal spores. Microwaves operate by exciting water molecules to produce heat and can only achieve temperatures up to the boiling point of water at 212°F (100°C).


Moreover, the substrate must have sufficient water content to be heated efficiently. Fungal spores contain low levels of water, making them challenging to heat, and steaming alone cannot produce high enough temperatures to kill them.


However, microwave steamers can be used to pasteurize small quantities of substrate and decrease the number of competing organisms.


What is approximate time frame for sterilizing mushroom substrate?


The time needed to sterilize mushroom substrate depends on the quantity of substrate and the temperature used. In general, sterilization of substrate blocks in a pressure cooker at 250°F (121°C) takes between 1 to 4 hours.


The minimum time needed to sterilize small blocks is one hour, and for larger blocks, 3 to 4 hours is sufficient. If the blocks are sterilized for too long, they may become over-sterilized.


After sterilization, it's important to let the substrate cool before inoculation, which can take up to 8 hours. When using atmospheric sterilization in a barrel steam sterilizer, it can take up to 18 hours to sterilize the substrate, and the substrate must also cool before inoculation.


Sterilizing mushroom substrate pros and cons



-Elimination of competing organisms

- increased yields through nutrient supplementation, and the ability to grow a wider variety of mushrooms on various nutrient-rich substrate


-high energy consumption

-higher cost of pressure equipment

-need for careful monitoring during the heating stage

-time-consuming process requires patience and dedication


The Bottom Line


Mushroom growers often use advanced equipment like pressure cookers, autoclaves, or barrel steam sterilizers to sterilize mushroom substrate. However, there's no need for expensive equipment or years of experience to start growing mushrooms at home. With simple methods and minimal equipment, you can grow gourmet mushroom species like shiitakes and oysters.

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